Spectrum data processing device

ABSTRACT

A mass spectrum generation section ( 22 ) generates an original mass spectrum at a peak of a target compound based on data acquired through analysis on a sample. A background spectrum generation section ( 23 ) generates a background spectrum in accordance with each of a plurality of predetermined background acquisition conditions. A difference mass spectrum calculation section ( 25 ) obtains a difference mass spectrum by subtracting the each background spectrum from the original mass spectrum, and a spectrum similarity calculation section ( 27 ) calculates a similarity between each difference mass spectrum and a standard mass spectrum of a candidate compound selected from a spectrum library ( 28 ). A compound identification section ( 29 ) identifies the candidate compound as the target compound if the highest similarity exceeds a predetermined threshold.

TECHNICAL FIELD

The present invention relates to a spectrum data processing device thatprocesses spectra data obtained in a sample measuring device, such as amass spectrometer, an absorption spectrometer, or a fluorescent X-rayspectrometer, and more particularly relates to a spectrum dataprocessing device that identifies a compound in a sample using aspectrum.

BACKGROUND ART

Identification of a compound in a sample using mass spectrometry isgenerally achieved by library search using a spectrum library storingstandard mass spectra of various compounds. Generally, in the librarysearch, a similarity in spectrum pattern between an actually measuredmass spectrum and a standard mass spectrum of each of the variouscompounds stored in the spectrum library is calculated. The actuallymeasured mass spectrum is obtained by performing mass spectrometry on asample including an unknown compound that is a target of identification.Then, the calculated similarity is compared with a predeterminedthreshold, or similarities of a plurality of candidate compounds arecompared with each other, and then a compound corresponding to the mostprobable standard mass spectrum is estimated as the unknown compound.

For a sample including a plurality of compounds, GC-MS or LC-MS is usedthat is a combination of the mass spectrometer and a gas chromatograph(GC) or a liquid chromatograph (LC). However, this solution is plaguedby a background superimposed on a mass spectrum of the target compound.The background corresponds to a mass spectrum derived from anothercompound (including contaminants). This occurs when the plurality ofcompounds cannot be completely separated from each other or thecontaminants are eluded at the same time point as the target compound.When the actually measured mass spectrum under an unignorable influenceof the background spectrum is used for the library search, the accuracyof the compound identification is compromised. In this context, PatentLiterature 1 discloses conventionally known background removalprocessing.

A device described in Patent Literature 1 executes processing ofsubtracting a background spectrum from an original mass spectrum ataround a point (retention time) corresponding to the peak of a targetcompound. A mass spectrum obtained in a time range with no peak of thetarget compound on a chromatogram is regarded as the backgroundspectrum. The time range with no peak of the target compound on thechromatogram includes, for example, a predetermined time range beforethe starting point of the peak of the target compound, and apredetermined time range after the end point of the peak of the targetcompound. Such processing is effective for removing a backgroundspectrum due to contaminants anticipated to be appearing substantiallyuniformly over the entire eluting time, such as contaminants mixed in amobile phase.

However, the processing is not suitable for removing a backgroundspectrum derived from another compound with the peak overlapping withthe peak of the target compound. When the peak of the other compound islikely to be overlapping with the peak of the target compound, thebackground spectrum needs to be estimated from a mass spectrum in a timerange (that is, a time range between the starting point and the endpoint of the peak) with the peak of the target compound appearing. Inview of this, the following algorithm is conventionally used forestimating the background spectrum. Specifically, first of all, acoefficient P is calculated by P=(tp−ts)/At, where tp, ts, and Δtrespectively represent a peak top time point, a peak starting timepoint, and a peak duration of a peak of the target compound on achromatogram. A mass spectrum is obtained by subtracting the massspectrum at the peak end point from the mass spectrum at the peakstarting point, and then is multiplied by the coefficient P. Then, aresult of adding the mass spectrum thus obtained to the mass spectrum atthe peak starting point ts is regarded as a background spectrum, and issubtracted from the original mass spectrum.

Patent Literature 2 discloses known background removal processingincluding: selecting a mass spectrum of a substance anticipated to be acontaminant overlapping with the target compound from a library; andsubtracting the mass spectrum from the original mass spectrum.

As described above, various algorithms are conventionally available forcalculating or estimating the background spectrum to be subtracted fromthe original mass spectrum. One conventional GC-MS (or LC-MS) dataprocessing device having a function for the background removalprocessing executes processing of automatically determining a time rangein which data used for calculating the background spectrum on thechromatogram is obtained. However, such automatic processing often leadsto library search resulting in inappropriate compound identification dueto a huge discrepancy between the background spectrum actuallysuperimposed on the original mass spectrum and the estimated backgroundspectrum resulting in a huge difference between the mass spectrum as aresult of subtraction of the background spectrum and the actual massspectrum of the target compound.

For example, in a case where the background spectrum may be calculatedby averaging a plurality of mass spectra obtained in a predeterminedtime range before the peak starting point of the target compound, whenanother compound is eluded at a high concentration around the peakstarting point of the target compound, the library search might resultin a spectrum similarity after the background subtraction being lowerthan that before the subtraction.

Thus, to effectively remove the background, an optimum algorithm needsto be selected for calculating the background spectrum for each peak ofthe target compound, or an appropriate time range needs to be determinedfor obtaining data used for calculating the background spectrum. Such anoptimum algorithm and time range are difficult to automaticallydetermine, and thus in actual cases, an operator is required to visuallycheck the waveform of a peak derived from the target compound on thechromatogram and a signal waveform around that time, to designate theappropriate algorithm and time range. Such work is extremely cumbersome,and takes so much time especially when a sample includes multiplecompounds to be identified.

The issues described above for the case of GC-MS or LC-MS using the massspectrometer similarly arise in a case of compound identification basedon an absorption spectrum in an LC with a detector such as photodiodearray detector (PDA detector) or an ultraviolet detector (UV detector),for example. The need for performing the background removal throughderivation of an appropriate background spectrum in accordance with thecurrent measurement condition and the like is not limited to GC or LC,and is totally relevant in a case where the compound identification isperformed based on a fluorescent X-ray spectrum obtained by afluorescent X-ray spectrometer or an infrared absorption spectrumobtained by a Fourier transform infrared spectrometer.

CITATION LIST Patent Literature

Patent Literature 1: JP 2000-206103 A

Patent Literature 2: JP 2006-38628 A (paragraph [0005])

SUMMARY OF INVENTION Technical Problem

The present invention is made to solve the problems described above, andis directed to providing a spectrum data processing device that canautomatically perform compound identification with high reliabilitybased on a spectrum obtained by subtracting an appropriate backgroundspectrum, without depending on an operation or determination by anoperator and the like.

Solution to Problem

A spectrum data processing device that processes spectra data obtainedby performing predetermined analysis on a sample including a compound,according to the present invention for solving the problem describedabove, includes:

a) a spectrum library configured to store a standard spectrum for eachcompound;

b) an original spectrum generation section configured to generate anoriginal spectrum of a target compound that is a target ofidentification based on spectra data obtained for a target sample;

c) a background spectrum acquisition section configured to acquire aplurality of background spectra in accordance with a plurality ofbackground acquisition conditions determined in advance;

d) a difference spectrum calculation section configured to calculate adifference spectrum obtained by subtracting each of a plurality ofbackground spectra, acquired by the background spectrum acquisitionsection, from the original spectrum; and

e) a compound identification section configured to calculate asimilarity between each of a plurality of the difference spectracalculated by the difference spectrum calculation section and one or aplurality of standard spectra stored in the spectrum library, and toidentify the target compound by referring to a plurality of thesimilarities thus calculated.

Examples of the spectra data that is the target of processing by thespectrum data processing device according to the present inventioninclude mass spectra data obtained by a mass spectrometer, absorptionspectrum data obtained by a PDA detector or a UV detector, infraredabsorption spectrum data obtained by a Fourier transform infrared (FTIR)spectrometer, and a fluorescent X-ray spectrum obtained by a fluorescentX-ray spectrometer. The cause of the background noise superimposed on aspectrum depends on the type of spectrometer or of spectrum data, andthus the background acquisition condition also depends on the type ofspectrometer or of spectrum data.

The original spectrum generation section of the spectrum data processingdevice according to the present invention generates the originalspectrum of the target compound that is an identification target andthus is unknown, based on the spectra data obtained for the targetsample. For example, when the spectra data that is the processing targetis mass spectra data repeatedly acquired over time by chromatograph massspectrometry, the total ion chromatogram may be generated based on suchdata and displayed on the screen of the display section. Then, the massspectrum at the retention time of the peak designated by the operator onthe total ion chromatogram display may be generated as the originalspectrum.

The background spectrum acquisition section acquires a plurality ofbackground spectra in accordance with a plurality of backgroundacquisition conditions determined in advance. Examples of the backgroundacquisition condition may include which data is used as the backgroundspectrum and how the background spectrum is generated from such data.

For example, when the spectra data that is the processing target is dataobtained by the chromatograph mass spectrometry, the background spectrumacquisition section may generate the background spectrum from a part ofdata other than the data used for generating the original mass spectrumor from data in a wide time rage including the data used for generatingthe original mass spectrum. The background spectrum may be generated notbased on the data obtained by performing predetermined analysis on thesampler including the target compound, and may be generated based ondata obtained by performing the predetermined analysis on a blank samplethat apparently does not include the target compound. A spectrumsatisfying a predetermined background acquisition condition may beselected, from multiple spectra stored in the spectrum library (oranother library), as the background spectrum. In any cases, a pluralityof background spectra are acquired in accordance with differentbackground acquisition conditions.

The difference spectrum calculation section calculates a differencespectrum by subtracting each of a plurality of background spectra fromthe original spectrum. The signal intensity of a negative value as aresult of the subtraction may be set to be 0. The background spectragenerated based on analysis results on different samples or acquiredfrom the spectrum library might be different from the original spectrumin detection sensitivity. In such a case, the subtraction processing maybe executed after processing such as normalizing the signal intensityvalue and the like for example may be executed to offset the differencein the detection sensitivity.

A difference spectrum obtained by subtracting a background spectrum witha spectrum shape close to that of the background spectrum actuallysuperimposed on the original spectrum relatively accurately reflects thespectrum of the target compound. Logically, a difference spectrumobtained by subtracting a background spectrum with a spectrum shapeclearly different from that of the background spectrum actuallysuperimposed on the original spectrum should be largely different fromthe spectrum of the target compound. The compound identification sectionat least calculates the similarity of each of the plurality ofdifference spectra to one or a plurality of standard spectra stored inthe spectrum library one by one.

The standard spectrum, that is, a compound for which the spectrumsimilarity is calculated is preferably selected as appropriate from thestandard spectra stored in the spectrum library. The selection may bemanually made with the operator in person selecting candidate compounds,estimated have a possibility of being the target compound, from thecompounds in the spectrum library. The compound information (such as theretention time, the retention index, the mass-to-charge ratio of themonitoring ion (such as a target ion or a qualifier ion), and aqualifier ion ratio) stored in the compound information storage sectionassociated with the spectrum library or the spectrum library may be usedfor automatically extracting a compound or a standard spectrum as apotential candidate from the spectrum library or the compoundinformation storage section. Any of these configurations involvingnarrowing down of the targets for which the spectrum similarity iscalculated can avoid the involvement of the compound that cannot be thetarget compound but accidentally has a high similarity to the standardspectrum.

The target ion, which is one type of the compound information, is an ioncharacterizing the corresponding compound, and the qualifier ion is anion also characterizing the corresponding compound but that is differentfrom the target ion and has a different mass-to-charge ratio. Thequalifier ion ratio is a relative ratio between the signal intensity atthe peak of the corresponding qualifier ion and the signal intensity atthe peak of the target ion. The retention index is similar in concept tothe retention time and is obtained by indexing the retention times ofcompounds to the retention time of a reference compound, such as acompound within the n-alkane homologue series.

The difference spectrum with a higher similarity more accuratelyreflects the spectrum of the target compound, and the standard spectruminvolved in the calculation of the similarity is likely to be thecompound corresponding to the target compound. Thus, for example, thecompound corresponding to the standard spectrum from which the highestone of a plurality of similarities calculated is identified as thetarget compound. Thus, highly reliable compound identification can beperformed without the need for the operator to designate an appropriatebackground acquisition condition.

The compound identification section, which calculates similaritiesbetween a plurality of difference spectra calculated by the differencespectrum calculation section and one or a plurality of standard spectrastored in the spectrum library, may further calculate the similaritybetween the original spectrum and one or a plurality of standard spectrato perform the compound identification by further referring to thissimilarity.

In a preferred aspect, the spectrum data processing device according tothe present invention further includes:

an initial similarity calculation section configured to calculate asimilarity between the original spectrum and the one or plurality ofstandard spectra stored in the spectrum library;

a similarity determination section configured to determine whether thesimilarity for each of the plurality of difference spectra calculated bythe compound identification section is smaller than the initialsimilarity calculated by the initial similarity calculation section, thesimilarity and the initial similarity being calculated using a same oneof the one or plurality of standard spectra; and an alert notificationsection configured to issue an alert to an operator, when the similaritydetermination section determines that at least one of the similaritiesof the plurality of difference spectra is smaller than the initialsimilarity, the similarities and the initial similarity being calculatedusing the same one of the one or plurality of standard spectra.

In another preferred aspect, the spectrum data processing deviceaccording to the present invention further includes:

an initial similarity determination section configured to calculate asimilarity between the original spectrum and the one or plurality ofstandard spectra stored in the spectrum library, and determines whetherthe similarity exceeds a predetermined threshold;

an individual similarity determination section configured to determinewhether the similarity for each of the plurality of difference spectracalculated by the compound identification section exceeds thepredetermined threshold; and

an alert notification section configured to issue an alert to anoperator, when the initial similarity determination section determinesthat the initial similarity exceeds the predetermined threshold and theindividual similarity determination section determines that thesimilarity does not exceed the predetermined threshold, the similarityand the individual similarity being calculated using a same one of theone or plurality of standard spectra.

In the two preferable aspects, the alert notification section may issuethe alert to the operator through a display for example. In theseaspects, the alert notification section issues the alert, when thesimilarity of the original spectrum, in the spectrum shape, to thestandard spectrum in the spectrum library is compromised as a result ofsubtracting the background spectrum. Such a situation might render thebackground removal difficult at least under a predetermined backgroundacquisition condition such as a case where another compound exists at ahigh concentration around the peak of the target compound on thechromatogram for example. Upon being notified of such a situation, theoperator can set an appropriate background acquisition condition by, forexample, checking the chromatogram in person or by the other like manualprocedure, to implement more accurate compound identification.

Advantageous Effects of Invention

With the spectrum data processing device according to the presentinvention, highly reliable compound identification can be automaticallyimplemented based on a difference spectrum as a result of obtainingsubtracting an appropriate background spectrum or on an originalspectrum without the background removal, without depending on anoperation or determination of an operator. Thus, for example, multiplecompounds in a sample can be identified efficiently with a lower workload on the operator. Furthermore, the configuration free of operationor determination by the operator can prevent the identification fromfluctuating depending on how much the operator is skilled orexperienced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a main part of oneembodiment of a GC-MS using a spectrum data processing device accordingto the present invention.

FIG. 2 is a diagram illustrating an example of a chromate gram obtainedin the GC-MS according to the embodiment.

FIG. 3 is a diagram illustrating background removal processing andcompound identification processing in the GC-MS according to theembodiment.

FIG. 4 is a diagram illustrating a configuration of a main part ofanother embodiment of a GC-MS using a spectrum data processing deviceaccording to the present invention.

FIG. 5 is a diagram illustrating background removal processing in aGC-MS according to yet another embodiment.

DESCRIPTION OF EMBODIMENTS

One embodiment of a gas chromatograph mass spectrometer (GC-MS)including a spectrum data processing device according to the presentinvention is described below with reference to the attached drawings.FIG. 1 is a diagram illustrating a configuration of a main part of theGC-MS according to the present embodiment. FIG. 2 is a diagramillustrating an example of a chromatogram obtained by the GC-MSaccording to the present embodiment. FIG. 3 is a diagram illustratingbackground removal processing and compound identification processing inthe GC-MS according to the present embodiment.

As illustrated in FIG. 1, a measurement section 1 includes a gaschromatograph (GC) section 11 and a mass spectrometry (MS) section 12.The GC section 11 includes unillustrated components such as a columnthat temporally separates compounds in a sample, a sample vaporizingchamber that is provided at an inlet end of the column and vaporizes aliquid sample so that the sample is sent into the column on a mobilephase (carrier gas), an injector that introduces a predetermined amountof the liquid sample into the sample vaporizing chamber, and a columnoven for controlling the temperature of the column. The MS section 12includes an ionization section that ionizes the compounds in sample gasas a result of passing through the column of the GC section 11, a massseparator such as a quadrupole mass filter that separates the generatedions in accordance with a mass-to-charge ratio m/z, and an ion detectorthat detects the ions separated in accordance with the mass-to-chargeratio. A detection signal obtained by the ion detector of the MS section12 is input to a data processing section 2 corresponding to the spectrumdata processing device according to the present invention.

The data processing section 2 includes functional blocks such as a datastorage section 20 receives the detection data and stores it in adigital format, a chromatogram generation section 21, a mass spectrumgeneration section 22, a background spectrum generation section 23, abackground acquisition condition storage section 24, a difference massspectrum calculation section 25, a standard spectrum selection section26, a spectrum similarity calculation section 27, a spectrum library 28,and a compound identification section 29. The spectrum library 28 storesstandard mass spectra in association with compound information aboutvarious compounds (such as a compound name, chemical formula, aretention time, a retention index, a mass-to-charge ratio of amonitoring ion (such as a target ion and a qualifier ion), and qualifierion ratio). The data processing section 2 is connected to an inputsection 3 and a display section 4 serving as a user interface.

The data processing section 2 may be formed of a general purposepersonal computer. The functions of the functional blocks describedabove may be implemented with dedicated data processing software,installed in the personal computer, executed on the computer.

The GC section 11 of the measurement section 1 in the GC-MS according tothe present embodiment separates various compounds in a sample in a timedirection, and sends sample gas including the separated compounds to theMS section 12. The MS section 12 repeats scan measurement over apredetermined mass-to-charge ratio range at a predetermined timeinterval, until a predetermined measurement time elapses after a timepoint when the sample is introduced to the GC section 11. Each scanmeasurement provides mass spectrum data indicating a change in a signalintensity over the predetermined mass-to-charge ratio range. As a resultof such GC/MS analysis, the data storage section 20 stores mass spectrumdata obtained for each measurement time point in the predetermined timerange.

Data processing for identifying an unknown target compound in a samplebased on the mass spectra data collected as described above is describedin detail.

A plurality of background acquisition conditions are stored in thebackground acquisition condition storage section 24 in advance. In thisexample, the following three background acquisition conditions [A] to[C] are stored.

[A] The background spectrum is determined as an average of a pluralityof mass spectra obtained in a predetermined time range (a range tb inFIG. 2 for example) before a starting time point ts of the peak of thetarget compound.

[B] The background spectrum is determined as an average of a pluralityof mass spectra obtain in a predetermined time range (a range tc in FIG.2 for example) at an after an end time point te of the peak of thetarget compound.

[C] A coefficient P is calculated by P=(tp−ts)/Δt, where tp, ts, and Δtrespectively represent a peak top time point, a peak starting timepoint, and a peak duration of the target compound. A mass spectrum isobtained by subtracting the mass spectrum at the peak starting point tsfrom the mass spectrum at the peak end point te, and then is multipliedby the coefficient P. Then, the resultant mass spectrum is added to themass spectrum at the peak starting point ts to be a background spectrum.

It is a matter of course that the background acquisition conditions arenot limited to those described above, and other background acquisitionconditions may be used that are different from those described above inan algorithm for generating the background spectrum and a time range inwhich data used for generating the background spectrum is obtained. Thebackground acquisition condition may be determined and stored in thebackground acquisition condition storage section 24 in advance by amanufacturer of the apparatus, or may be determined and stored in thebackground acquisition condition storage section 24 as appropriate by auser that has purchased the apparatus.

For example, when an operator performs a predetermined operation on theinput section 3, the chromatogram generation section 21 generates atotal ion chromatogram (TIC) based on the mass spectra data over theentire measurement time stored in the data storage section 20 asdescribed above, and displays the TIC on a screen of the display section4. The operator checks the TIC on the screen, and designates the targetpeak, in peaks on the TIC, for which the compound identification is tobe performed. FIG. 2 illustrates a part of the TIC, which suffices ifthe target peak can be designated by performing a clicking operation ona peak on the TIC using a pointing device, for example. The designationby the operator should not be construed in a limiting sense, and thecompound identification may be performed completely automatically on apeak detected automatically in accordance with a predetermined algorithmon the TIC. Furthermore, an extracted ion chromatogram (masschromatogram) of monitoring ions of candidate compounds may be usedinstead of the TIC.

The mass spectrum generation section 22 reads data corresponding to thetime point tp of the peak top of the designated peak from the datastorage section 20 to generate a mass spectrum. This serves as theoriginal mass spectrum of the target compound (see FIG. 3A). Generally,a background spectrum due to various factors is superimposed on theoriginal mass spectrum. Examples of the factors include contaminantsderived from the mobile phase used in the GC section 11 and othercompounds overlapped due to insufficient separation in the GC section11.

The background spectrum generation section 23 reads out a plurality ofbackground acquisition conditions stored in advance in the backgroundacquisition condition storage section 24. Then, required data is readout from the data storage section 20 in accordance with each backgroundacquisition condition to generate a background spectrum. Note that thebackground spectrum may be generated by using background acquisitionconditions appropriately selected by the operator from all thebackground conditions stored in the background acquisition conditionstorage section 24, instead of using all the background acquisitionconditions.

In this example, the background spectrum is generated based on each ofthe three background acquisition conditions [A] to [C] (see FIG. 3B).Generally, different background acquisition conditions result inbackground spectra with different spectrum shapes.

Next, the difference mass spectrum calculation section 25 calculatesdifference mass spectra as a result of subtracting each of a pluralityof background spectra from the original mass spectrum of the target peak(see FIG. 3C). Subtraction processing for two mass spectra may beperformed in a conventional manner. Specifically, the signal intensitymay be subtracted for each mass-to-charge ratio, and the signalintensity resulting in a subtraction result of a negative value may beset to be 0. As described above, different spectrum shapes of backgroundspectra result in calculated mass spectra with different spectrumshapes. If the background spectrum truly reflects the backgroundsuperimposed on the original mass spectrum, the difference mass spectrumis simply equivalent to the mass spectrum of the target compound. Such amass spectrum of the target compound is supposed to substantially matchthe standard mass spectrum corresponding to the candidate compoundstored in the spectrum library 28, expect for the case where the targetcompound is not stored in the spectrum library 28. However, it is notvery peculiar for the spectrum library 28 to store a compound that iscompletely different from the target compound but has a spectrum patternthat accidentally resembles that of the difference mass spectrum derivedfrom the target compound.

Thus, the standard spectrum selection section 26 selects the standardmass spectrum (spectra) of one or a plurality of candidate compoundsthat could be the target compound, from multiple standard mass spectrastored in the spectrum library 28. The selection may be made based ondesignation by the operator through the input section 3. The retentiontime tr corresponding to the target compound to be identified may beobtained from the TIC or an extracted ion chromatogram of the monitoringions of the target compound, and a compound, in the multiple compoundsstored in the spectrum library 28, with the retention time within apredetermined allowable range relative to the retention time tr, may beextracted. Then, a standard spectrum of the extracted compound may beacquired. This may result in one or a plurality of standard mass spectraof the candidate compound(s) obtained. Here, a retention index may beused instead of the retention time.

The spectrum similarity calculation section 27 sequentially compares theone or a plurality of standard mass spectra selected from the spectrumlibrary 28 one by one with the original mass spectrum and eachdifference mass spectrum, to calculate each similarity reflecting howsimilar one is to the other in the spectrum shape The similarity may becalculated by a method that is the same as that used in the compoundidentification through conventional library search.

When there are three difference mass spectra as illustrated in FIG. 3C,four similarities of the three difference mass spectra and the originalmass spectrum are calculated for the standard mass spectrum of eachcandidate compound, and thus four similarities S1, S2, S3, and S4 areobtained as illustrated in FIG. 3D. When there are further candidatecompounds, a plurality (four in the example described above) ofsimilarities are calculated for the standard mass spectrum of each ofthe further candidate compounds.

Then, the compound identification section 29 performs the compoundidentification by referring to the plurality of similarities calculated.Specifically, if there is only one candidate compound, one with themaximum value is picked up from all the similarities calculated, andwhether the value of the highest similarity exceeds a predeterminedthreshold is determined. If the predetermined threshold is exceeded, thesingle candidate compound corresponding to the standard mass spectrum isidentified as the target compound, and if not, the result of theidentification may be determined as unidentifiable (or lowidentification reliability). If there are a plurality of candidatecompounds, one with the maximum value is picked up from all thesimilarities calculated, and whether the value of the highest similarityexceeds a predetermined threshold is determined. If the predeterminedthreshold is exceeded, the candidate compound corresponding to thestandard mass spectrum with the highest similarity is identified as thetarget compound. Of course, if the highest similarity does not exceedthe predetermined threshold, the result of the identification may bedetermined as unidentifiable (or low identification reliability) also inthis case.

When the compound is successfully identified in the manner describedabove, the compound identification section 29 displays theidentification result on the screen of the display section 4. When it isdetermined unidentifiable, such a result is displayed.

As described above, the GC-MS according to the present embodiment canperform compound identification based on a result of subtracting thebackground spectrum obtained under the most appropriate backgroundacquisition condition from the original spectrum automatically, withoutthe need for the operator to go through cumbersome operations ofvisually checking the TIC and manually designating an appropriatebackground acquisition condition.

In the description above, the spectrum similarity calculation section 27not only calculates the similarity between the difference mass spectrumand the standard mass spectrum but also calculates the similaritybetween the original mass spectrum and the standard mass spectrum, to bereferred to by the compound identification section 29 for performing thecompound identification. This can be modified so that the similaritybetween the original mass spectrum and the standard mass spectrum is notused for the compound identification.

Next, a GC-MS according to another embodiment featuring data processingslightly different from that of the GC-MS according to the embodimentdescribed above is described with reference to FIG. 4.

FIG. 4 is a functional block diagram of a data processing section 200 inthe GC-MS according to the present embodiment. The GC-MS has themeasurement section 1 that is the same as that in the embodimentillustrated in FIG. 1. Components in FIG. 4 with the same referencenumerals in FIG. 1 has basically the same configurations as thecounterparts. The GC-MS of this embodiment includes a data processingsection 200 including the components of the data processing section 200of the GC-MS of the above described embodiment, and further includes aninitial similarity calculation section 201, a similarity determinationsection 202, and an alert notification section 203.

In the GC-MS according to the present embodiment, the initial similaritycalculation section 201 calculates a similarity between the originalmass spectrum of the target peak and each of a plurality of standardmass spectra stored in the spectrum library 28. The similaritydetermination section 202 compares the level of similarity between theoriginal mass spectrum and a standard mass spectrum with the level ofsimilarity of each of a plurality of difference mass spectra and thesame standard mass spectrum. The similarity obtained with the originalmass spectrum having a higher level than the similarity obtained withthe difference mass spectrum indicates that the background removalcompromised the similarity to the standard mass spectrum in the spectrumshape. This means that the background spectrum is not appropriatelyobtained.

For example, such an incident occurs when another compound is eluded ata high concentration around the starting point of the target peak in acase where the background spectrum is generated under the backgroundacquisition condition [A] described above. Thus, when the similarityobtained with the original mass spectrum has a higher level than thesimilarity obtained with the difference mass spectrum, the alertnotification section 203 displays an alert display indicating that thebackground acquisition condition might be inappropriate on the screen ofthe display section 4. The alert display may be displayed together withthe identification result of the normal compound identificationprocessing under progress. In response to this alert display, theoperator may check the TIC and the like on the screen and may manuallyset the background acquisition condition as appropriate.

The similarity determination section 202 may determine whether or notthe initial similarity between the original mass spectrum a standardspectrum exceeds a predetermined threshold, and determine whether or notthe similarities between the same standard spectrum and a plurality ofdifference mass spectra exceed the predetermined threshold. Then, whenthe initial similarity exceeding predetermined threshold drops to orbelow the threshold as a result of the background removal, the alertnotification section 203 may display the alert display, indicating thatthe background acquisition condition might be inappropriate, on thescreen of the display section 4.

Such additional processing can avoid erroneous compound identificationdue to the operator being in charge of an operation and determination ina case where an appropriate background spectrum for performing thebackground removal fails to be automatically acquired.

The GC-MS according to the embodiment described above generates abackground spectrum by using a part of data obtained by performing GC/MSanalysis on a sample including a target compound. Alternatively, thebackground spectrum may be generated from another type of data that isnot derived from the sample. FIG. 5 is a diagram illustrating an exampleof a case where subtraction processing is executed with a backgroundspectrum generated from the other type of data. Here, when the TIC isgenerated based on the data about the target sample with the matrix(main component) of the target sample and the target compound in thesample being close to each other in the retention time, for example, thepeak derived from the matrix and the peak of the target compound overlapas illustrated in the left section of FIG. 5A, and the original massspectrum includes both of the peaks as illustrated in the right sectionin FIG. 5A.

In such a case, the background spectrum may be generated from dataobtained as a result of GC/MS analysis on a blank sample including thematrix only and not including the target compound. In this context,background acquisition conditions set for acquiring such a backgroundspectrum may include using data as a result of the GC/MS analysis on theblank sample and obtaining a mass spectrum in a retention time aroundthe peak top of the peak derived from the matrix. For example, the leftsection in FIG. 5B illustrates a TIC generated based on data obtained asa result of the GC/MS analysis on the blank sample and the right sectionin FIG. 5B illustrates the mass spectrum of the peak derived from thematrix in the TIC, that is, the background spectrum. When thisbackground spectrum is subtracted from the original mass spectrum, themass spectrum peak derived from the spectrum peak is removed so that thepeak derived from the target compound mainly remains, resulting in ahigh similarity to the standard mass spectrum.

The spectrum data processing device according to the present inventionis not limited to the processing on the mass spectra data obtained bythe GC-MS, and may also be used for compound identification based onmass spectra data obtained by an imaging mass spectrometer for examplewhich involves no combination with GC or LC.

The present invention is not limited to the mass spectra data obtainedby the mass spectrometer and may be applied to a case of processingabsorption spectra data repeatedly obtained by a PDA detector or a UVdetector used in an LC as the detector. Furthermore, the spectrum dataprocessing device according to the present invention may be used forspectrometers in general that identify a compound (or an element)included in a sample through library search using a spectrum, such as afluorescent X-ray spectrum obtained by a fluorescent X-ray spectrometer.

The embodiments and the modifications described above are merelyexamples of the present invention, and thus modification, addition, andcorrection to the embodiments and the modifications without departingfrom the gist of the present invention are apparently included in thescope of the claims of the present application.

REFERENCE SIGNS LIST

-   1 . . . Measurement Section-   11 . . . Gas Chromatograph (GC) Section-   12 . . . Mass Spectrometry (MS) Section-   2, 200 . . . Data Processing Section-   20 . . . Data Storage Section-   21 . . . Chromatogram Generation Section-   22 . . . Mass Spectrum Generation Section-   23 . . . Background Spectrum Generation Section-   24 . . . Background Acquisition Condition Storage Section-   25 . . . Difference Mass Spectrum Calculation Section-   26 . . . Standard Spectrum Selection Section-   27 . . . Spectrum Similarity Calculation Section-   28 . . . Spectrum Library-   29 . . . Compound Identification Section-   201 . . . Initial Similarity Calculation Section-   202 . . . Similarity Determination Section-   203 . . . Alert Notification Section-   3 . . . Input Section-   4 . . . Display Section

1. A spectrum data processing device that processes spectra data obtained by performing predetermined analysis on a sample including a compound, the spectrum data processing device comprising: a) a spectrum library configured to store a standard spectrum for each compound; b) an original spectrum generation section configured to generate an original spectrum of a target compound that is a target of identification based on spectra data obtained for a target sample; c) a background spectrum acquisition section configured to acquire a plurality of background spectra in accordance with a plurality of background acquisition conditions determined in advance; d) a difference spectrum calculation section configured to calculate a difference spectrum obtained by subtracting each of a plurality of background spectra, acquired by the background spectrum acquisition section, from the original spectrum; and e) a compound identification section configured to calculate a similarity between each of a plurality of the difference spectra calculated by the difference spectrum calculation section and one or a plurality of standard spectra stored in the spectrum library, and to identify the target compound by referring to a plurality of the similarities thus calculated.
 2. The spectrum data processing device according to claim 1, further comprising: an initial similarity calculation section configured to calculate a similarity between the original spectrum and the one or plurality of standard spectra stored in the spectrum library; a similarity determination section configured to determine whether the similarity for each of the plurality of difference spectra calculated by the compound identification section is smaller than the initial similarity calculated by the initial similarity calculation section, the similarity and the initial similarity being calculated using a same one of the one or plurality of standard spectra; and an alert notification section configured to issue an alert to an operator, when the similarity determination section determines that at least one of the similarities of the plurality of difference spectra is smaller than the initial similarity, the similarities and the initial similarity being calculated using the same one of the one or plurality of standard spectra.
 3. The spectrum data processing device according to claim 1, further comprising: an initial similarity determination section configured to calculate a similarity between the original spectrum and the one or plurality of standard spectra stored in the spectrum library, and determines whether the similarity exceeds a predetermined threshold; an individual similarity determination section configured to determine whether the similarity for each of the plurality of difference spectra calculated by the compound identification section exceeds the predetermined threshold; and an alert notification section configured to issue an alert to an operator, when the initial similarity determination section determines that the initial similarity exceeds the predetermined threshold and the individual similarity determination section determines that the similarity does not exceed the predetermined threshold, the similarity and the individual similarity being calculated using a same one of the one or plurality of standard spectra.
 4. The spectrum data processing device according to claim 1, wherein the spectra data that is a target of processing is mass spectra data repeatedly acquired over time through chromatograph mass spectrometry.
 5. The spectrum data processing device according to claim 1, further comprising a compound information storage section configured to store compound information about each standard spectrum including at least one of a retention time, a retention index, a mass-to-charge ratio of a monitoring ion including a target ion and/or a qualifier ion, and a qualifier ion ratio, wherein the compound identification section automatically selects one or a plurality of standard spectra used for similarity calculation, through narrowing down using one or a plurality of pieces of the compound information. 